This invention relates to passive detection and identification of a source of electromagnetic radiation and, more particularly, to detection of hot gasses undergoing rapid motion, as in a rocket plume, by examination of spectral line broadening due to particle collisions, and by examination of Doppler shifting of the spectral lines due to particle motion in a known emission spectrum of the constituent gasses of a plume, thereby to identify the source of the radiation.
Various objects, such as the plume of a rocket or fire, or gas discharge of a lamp, are known to act as sources of radiation having characteristic spectra. There are situations in which it is desirable to detect and/or identify such a source from a viewing site distant from the source. However, a problem arises in that the usual apparatuses for detecting and identifying such a source, or target, such as active radar by way of example, are not operative with the foregoing type of radiant energy signal for a passive determination of the presence and identity of the source.